Catalysts for selective conversion of nitric oxide to ammonia (NTA) with propene in the presence of a large excess of oxygen and water vapor

Abstract

Catalysts for the NO to NH₃ reaction in the presence of 10% O₂ and 10% H₂O were investigated on a TiO₂(rutile) support. The activity sequence of the various metals supported at 2 wt% was Ag > Ni > Zn > Sn = Bi > In > others (22 elements in total). The Ag/TiO₂ catalyst with the highest activity was further investigated to increase and stabilize the activity. The metallic silver particles prepared by ethanol reduction of Ag in solution were supported on TiO₂ (ET method). Silver was supported as fine particles, and the activity was significantly improved. However, repeated use of the ET catalysts at 500–300 °C resulted in Ag agglomeration and a significant decrease in activity. In order to prevent activity degradation at high temperatures, the catalyst was used only at temperatures below 400 °C, and stable activity was obtained. The addition of 0.25% C₃H₆ was found to be sufficient to convert half of 0.1% NO to NH₃ on 1.0AgET/TiO₂. To stabilize the catalytic activity of AgET/TiO₂, Nd was loaded onto the TiO₂ support prior to Ag loading (ND method). The loading of 1–5 mol% Nd resulted in the formation of new pores with a diameter of 14 nm. 1.0AgET/1NdTiO₂ gave higher ammonia yields than 1.0AgET/5NdTiO₂, and was more stable. The dependence on Ag loading (0.7–5.0 wt%) on 1NdTiO₂ was measured and it was found that the catalyst loaded with 1.0 wt% Ag gave the highest activity. The activity change in repeated experiments was very small for all the ND-ET catalysts. The NH₃ yield reached 88% under the reaction conditions of 0.1% NO, 0.5% C₃H₆, 10% O₂, 10% H₂O, balanced N₂, and a space velocity of 10 000 h⁻¹. Finally, the activities for the C₃H₆ and NH₃ combustion were measured. NH₃ can be produced at the temperature of C₃H₆ combustion and consumed at the temperature of NH₃ combustion. Due to the Ag loading, the temperature for C₃H₆ combustion became much lower than that for NH₃ combustion, indicating that the NH₃ formation activity was greatly enhanced by supporting Ag. It was also found that the NH₃ combustion activity of AgET/TiO₂ became higher on the catalysts used repeatedly, resulting in a narrowing of the ammonia production temperature range. The effect of reducing gas species was also investigated.